TSPD: Register preempted SMC error code with EHF

An earlier patch extended ehf_allow_ns_preemption() API to also register
an error code to offer to Non-secure when a Yielding SMC is preempted by
SDEI interrupt. In TSPD's case, register the error code TSP_PREEMPTED.

Change-Id: I31992b6651f80694e83bc5092b044ef7a3eda690
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>